1. Field of the Invention
The present invention relates to a master drive that is connected to a slave device via a serial communication bus for carrying out data transmission and reception in order to control a motor connected via an inverter to a DC link part, which is connected to an alternating-current power source via a converter.
2. Description of Related Art
A master device that is connected to a slave device via a serial communication bus for carrying out data transmission and reception in order to control a motor in a system, which includes motors for driving a feed axis and a main axis of a machine tool, an industrial robot arm, etc., is proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2008-242728 (JP2008-242728A).
In such a system which includes the motors, the master device is comprised of a converter control device configured to control a converter in order to convert alternating-current power supplied from an alternating-current power source into direct-current power, an upper control device such as a CNC (computer numerical control), etc., and the slave device is implemented by an inverter control device configured to control an inverter in order to convert the direct-current power which is converted by a converter into alternating-current power, etc. For example, in the case where the master device is the converter control device and the slave device is the inverter control device, data such as status information, alarm information, etc., is shared between the converter control device and the inverter control device by carrying out data transmission and reception via a serial communication bus. In the case where the master device is the upper control device and the slave device is the inverter control device, data such as motor position or speed command data, etc., is transmitted from the upper control device to the inverter control device via the serial communication bus and data such as current value data, motor position or speed data, etc., is transmitted from the inverter control device to the upper control device via the serial communication bus, thereby the data is shared between the converter control device and the inverter control device.
A serial communication among a plurality of devices can transmit and receive an amount of data larger than the amount of data that can be transmitted and received by parallel communication with electric lines in a number smaller than the number of electric lines used for the parallel communication (for example, two electric lines are used in the case of I2C communication).
In the serial communication, if it is desired to increase the amount of data to be transmitted and received per unit time, it is necessary to increase the data communication speed, i.e., extend the bandwidth of a signal necessary to transmit data. In the case where the bandwidth of the signal necessary to transmit data is extended, it is also necessary to extend the bandwidth of a filter provided in at least one of the master device and the slave device in order to remove noise received in the serial communication. Therefore, as the bandwidth of the filter is extended, the influence of noise received in the serial communication increases.
Measures against the noise received in the serial communication are taken in order to extend the bandwidth of the signal necessary to transmit data and to maintain the serial communication with high reliability. As the measures against the noise received in the serial communication, there are an increase of the thickness of the cable used for the serial communication in accordance with an increase of the data communication speed in order to reduce an impedance of the cable used for the serial communication, an increase of the thickness of the shield for the cable used for the serial communication in accordance with an increase of the data communication speed in order to prevent fluctuations in the electric potential due to an external influence, a reduction of the length of the cable used for the serial communication in accordance with an increase of the data communication speed in order to reduce the impedance of the cable used for the serial communication, etc.
Consequently, the cost necessary to take measures against the noise received in the serial communication and restrictions on the length of the cable used for the serial communication increase as the data communication speed increases.
When controlling the motor, it is desirable to reduce the data transmission and reception time interval necessary to complete the data transmission and reception of data necessary to drive the motor between the master device and the plurality of slave devices through the serial communication bus as much as possible (for example, 1 msec) during the period from the time when the master device is activated to the time when the preparation to drive the motor is completed, in order to reduce the period from the time when the master device is activated to the time when the preparation to drive the motor is completed (period required to start-up the motor). There is an influence of switching of a switching element, which is included in equipment (for example, inverter) controlled by at least one of the master device and the slave devices, during the period from the time when the preparation to drive the motor is completed to the time when the motor is stopped, however, there is no such influence during the period from the time when the master device is activated to the time when the preparation to drive the motor is completed. Because of this, the noise received in serial communication during the period from the time when the master device is activated to the time when the preparation to drive the motor is completed is remarkably small compared to the noise received in serial communication during the period from the time when the preparation to drive the motor is completed to the time when the motor is stopped. Consequently, it is possible to set a first communication speed during the period from the time when the master device is activated to the time when the preparation to drive the motor is completed higher than a second communication speed during the period from the time when the preparation to drive the motor is completed to the time when the motor is stopped, in order to reduce the period required to start-up the motor as much as possible.
On the other hand, the data transmission and reception period necessary to periodically repeat data transmission and reception between the master device and the slave devices through the serial communication bus during the period from the time when the preparation to drive the motor is completed to the time when the motor is stopped, is fixed. Further, such a data transmission and reception period may be longer (for example, 1 sec) than the above-mentioned data transmission and reception time interval. In a severe noise environment in which there is an influence of switching of the switching element during the period from the time when the preparation to drive the motor is completed to the time when the motor is stopped, priority is given to secure data transmission and reception over fast data transmission and reception. Consequently, it is desirable that the above-mentioned second communication speed is lower than the above-mentioned first communication speed.
In the conventional motor control, the first communication speed is set to be equal to the second communication speed. That is, the data communication speed of data transmitted and received between the master device and the slave devices via the serial communication bus is fixed during the period from the time when the master device is activated to the time when the master device is stopped after the drive and stop of the motor are repeated. However, the cost necessary to take measures against the noise in serial communication and restrictions on the length of the cable used for serial communication is increased as the data communication speed increases in order to reduce the period required to start-up the motor. On the other hand, the period required to start-up the motor increases as the data communication speed is decreased in order to reduce the cost necessary to take measures against the noise in serial communication and restrictions on the length of the cable used for serial communication.
Consequently, in conventional motor control, it is difficult to reduce the period required to start-up the motor and at the same time, to reduce the cost necessary to take measures against the noise in serial communication and restrictions on the length of the cable used for serial communication.